The invention relates to drive torque control for a vehicle and, in particular, to a drive torque control method and a drive torque control device capable of controlling a wheelie as a phenomenon in which a front wheel comes off the ground during acceleration as well as to a vehicle including the same.
A vehicle, for example, a typical two-wheeled vehicle (a motorcycle) has a rear wheel as a drive wheel, and a vehicle body is accelerated by the rear wheel. At this time, due to a positional relationship between a point at which the rear wheel contacts a road surface and center of gravity of the vehicle body, specified moment is generated in terms of the gravity. This moment that is generated during acceleration is moment that acts in a direction to cause a front wheel of the two-wheeled vehicle to come off the ground. Accordingly, when an abrupt accelerating operation is performed during traveling of the two-wheeled vehicle, the front wheel may come off the ground. Such a phenomenon is generally referred to as a “wheelie”.
In addition to the case where the abrupt accelerating operation is performed, the wheelie also occurs in a case where a clutch is abruptly connected at a start or the like. Meanwhile, many of the two-wheeled vehicles that are available in the market in recent years are lightweight and high-powered. Thus, the wheelie is more likely to occur to the latest high-output two-wheeled vehicles than to conventional two-wheeled vehicles. In particular, this tendency is more intense for motocross bikes and supermoto bikes which are lightweight and each of which has the vehicle body with the high center of gravity. When the wheelie occurs, the vehicle body becomes unstable. Thus, a typical rider tries to reduce the wheelie by reducing an operation amount of an accelerator or by actuating a rear brake.
However, the wheelie suddenly occurs depending on a condition, and the typical rider may not be able to appropriately handle the wheelie. For this reason, drive torque control that can prevent the wheelie or terminate the wheelie in an early stage regardless of the rider's intention has been suggested. In order to execute the drive torque control for reducing the wheelie, the wheelie needs to be detected as a precondition. These methods for detecting the wheelie and executing the drive torque control are largely divided into two categories. One is based on traction control, and the other is based on information from various sensors that are attached to the vehicle body.
First, in the method based on the traction control, feedback control is executed on the basis of a difference between front and rear wheel speeds, and the wheelie is thereby reduced. More specifically, the front wheel remains off the ground while the wheelie occurs. Thus, the front wheel speed is reduced, and a speed difference thereof from the rear wheel speed is generated. Based on this speed difference, drive torque (that is, engine output torque) of the rear wheel is reduced.
Next, in the method using the information from the various sensors, a wheel speed sensor, an acceleration sensor, a suspension stroke sensor, an angular speed (gyroscopic) sensor, and the like are used. The acceleration sensor detects acceleration of the two-wheeled vehicle with respect to the road surface. The acceleration is combined with the information, such as a position of the center of gravity of a vehicle body and a vehicle body weight, and, when the acceleration exceeds a specified value, the occurrence of the wheelie can be predicted. In addition, the wheel speed sensor detects a wheel speed, and the acceleration of the vehicle body can be computed from the wheel speed. Thus, similar to the case of the acceleration sensor, there is a case where the wheelie can be detected.
Furthermore, the suspension stroke sensor can detect a stroke (an expanding amount or a contracting amount) of a suspension. When the wheelie occurs, an external force from the road surface is no longer applied to a front fork. Thus, the stroke in the maximum length is detected. In this way, the wheelie is detected. Moreover, the angular speed sensor is a sensor for detecting angular speeds around the center of gravity of the vehicle body. Of these, a pitch angular speed is used to detect the occurrence of the wheelie. When the wheelie occurs, the drive torque (that is, the engine output torque) of the rear wheel is reduced on the basis of at least one type of the information from these sensors. As a specific example, a wheelie state is detected by the angular speed sensor, and engine output reduction control is executed in accordance with a degree of the wheelie (see JP-A-2010-229912).
FIG. 6 includes schematic charts for explaining conventional drive torque control that is executed in a case of the wheelie. In FIG. 6(A), a horizontal axis indicates time, and a vertical axis indicates the wheel speeds of the front wheel and the rear wheel. Here, a one-dot chain line represents a wheel speed 101F of the front wheel, and a broken line represents a wheel speed 101R of the rear wheel. In addition, in FIG. 6(B), a horizontal axis indicates the time, and a vertical axis indicates the drive torque of the rear wheel. Here, a broken line represents a drive torque request 111D from a rider, and a one-dot chain line represents drive torque 111R that is actually applied to the rear wheel. In these charts, the wheelie occurs at time T1, and the wheelie ends at time T3. As it is understood from FIG. 6(A), the wheel speed 101F of the front wheel is increased in the same manner as the wheel speed of the rear wheel until the time T1, and is reduced from the time T1 to time T2. This is because the front wheel comes off the road surface due to the wheelie and rotary power from the road surface that is generated by traveling is no longer generated. On the other hand, because the rear wheel is the drive wheel, the wheel speed 101R is increased in proportion to an increase in a vehicle speed.
In such a case, irrespective of the drive torque request 111D from the rider, the drive torque 111R that is actually applied to the rear wheel is reduced at the time T1, at which the wheelie occurs. Then, at the time T2 at which the wheelie ends, the reduced drive torque 111R is controlled in a direction of an increase. Thereafter, a specified period is required for the actual drive torque 111R of the rear wheel to reach the same level as the drive torque request 111D from the rider.